One type of a videotex display system is what is commonly referred to as the NAPLPS system. NAPLPS (North American Presentation Level Protocol Syntax), is a videotex standard in the United States based on TELIDON which is the Canadian standard. The NAPLPS protocol is published by the American National Standards Institute and the Canadian Standards Association. A detailed explanation can be found in the publication: "CSA T500-198x ANSI BSR x 3.110-198x, Sep. 9, 1983," by the American National Standards Institute and the Canadian Standards Association, at pages 11 to 17, beginning at line 11.
In the NAPLPS system, a graphical figure is transmitted and received by a method that is generally referred to as an alphageometric system. Specifically, all graphical figures are expressed by a combination of dots, lines, arcs, squares and polygons. From the transmission side, a code, generally referred to as a picture description instruction (PDI) code, is used to specify the type, position and size of the graphical figure to be transmitted. On the reception side, the PDI code is received and decoded to cause the terminal to generate sufficient dots and at the correct locations on its display screen to display the original graphical figure on a CRT display. A salient characteristic of the NAPLPS videotex system is that the conveyed display is terminal independent, i.e., the transmitter of the display message does not have to take into account the display resolution capability of the receiving terminal. A prior art terminal of this type is illustrated in U.S. Pat. No. 4,439,761 and U.S. Pat. No. 4,439,759.
When "a rising-sun flag" is drawn on the entire video display screen, for example, the necessary PDI code is defined as follows:
______________________________________ PDI code Meaning Display Drawing ______________________________________ RESET Reset picture Clear picture screen screen SET-COLOR Set color as Figure and white white character drawn hereinafter become white POINT SET Set current FIG. 6A 0/0 position at left-hand side corner (0,0) RECTANGLE- Draw rectangle White rectangle FIG. 6B FILL of 1 wide and is drawn on 1,0.75 0.75 long, whole picture Paint out the screen inside SET-COLOR Set color as Figure and red red character will be drawn in red POINT SET Set current Center point FIG. 6C 0.3,0.375 position on lefthand side of picture screen ARC-FILL Draw arc (in Both ends of FIG. 6D 0.7,0.375 this case, diameter are 0.3,0.375 circle) and specified and paint out the circle is drawn inside in red POINT SET Set current Under side on 0.4,0.1 position picture screen SI Shift-in Will be treated not as PDI but as character thereafter JAPAN character, FIG. 6E JAPAN is drawn SO Shift-out Returned to PDI ______________________________________
As just described in the above-mentioned description, the PDI code indicates the position and relative size of the graphical figure. The number of dots necessary to present this picture are a function of the resolution capacity of the terminal's display and are determined by the terminal's controller. The values which correspond to the PDI code are those of the normalized coordinates. These values are then shown on a video screen 1 of the CRT display 34, as best illustrated in FIG. 5.
In the NAPLPS system, the resolution of the display is determined by the resolution capability or normalization of a user's terminal apparatus. For example, even if the graphical figure is transmitted for a resolution of 4096 dots per line, which is the highest resolution available, a user's terminal apparatus must be of the same capability as that of the transmitter in order to display such a high resolution picture. If the user's terminal apparatus is capable of displaying only 256 dots, a low resolution picture is displayed by only displaying a fraction of these dots, e.g., only every 16th dot for a standard TV display. If the user's terminal apparatus, however, has a high resolution capability, the entire 4096 dots can be displayed.
A serious problem, however, is present in prior art systems. That problem is the incompatability of the NTSC and PAL systems; The NTSC system uses 525 scanning lines and the PAL system uses 625 scanning lines. As shown in FIG. 7A, if the video display screen of a CRT display utilizes the NTSC system, then there are 256 displayable dots in the horizontal direction and 200 displayable dots in the vertical direction. As shown in FIG. 7B, a video display screen utilizing the PAL system has 256 dots in the horizontal direction and 240 dots in the vertical direction.
Thus, if the PDI code is decoded without further processing, the display data will cause the graphical picture displayed on the CRT display to be compressed or expanded in the vertical direction depending on the type of system used.